1. Field of the Invention
This invention relates to the field of electrochemistry, and more particularly relates to thermally activated electrochemical cells having an oxyanionic electrolyte containing novel cathode materials, resulting in an increase in cell potential.
2. Description of the Prior Art
Thermally activated electrochemical cells or batteries have been used quite extensively in military applications, such as a power source for arming devices, because of their long shelf life and compactness, and capability of withstanding shock and vibration. Batteries of this type typically include an electrolyte which, under normal storage condictions, is solid and does not conduct electricity. When the battery and/or the electrolyte is heated to a predetermined temperature, as by igniting a built-in pyrotechnic heat source such as an electric match, squib or percussion primer, the electrolyte, upon changing to a molten state, becomes conductive and ionically connects the electrodes to provide the desired electromotive force.
Nitrate salts have been proposed for use in thermal batteries because of their low melting points. See U.S. Pat. No. 4,260,667 to Miles and Fletcher. For example, potassium nitrate-lithium nitrate (KNO.sub.3 -LiNO.sub.3) mixtures melt at temperatures as low as 124.degree. C. The use of a lower melting electrolyte can shorten a battery's activation time and reduce the weight of heat sources and insulation.
A particular problem area of thermal cells using oxidizing molten salts such as molten nitrates or molten perchlorates as cathode materials is the slow kinetics for the reduction of the oxyanion. For example: EQU NO.sub.3.sup.- +2e.sup.- .fwdarw.NO.sub.2 +0.sup.= EQU C10.sub.4.sup.- +2e.sup.- .fwdarw.C10.sub.3 +0.sup.=
The large overpotentials for the reduction of these oxyanions results in significant lowering of cell potentials at moderate current densities. Consequently, silver salts have been added to the electrolytes as cathode materials to improve cell potentials. These silver salts involve the reduction of the metal ions to the free metal in reversible electrode reactions, such as: EQU AgNO.sub.3 +e-.revreaction.Ag+NO.sub.3.sup.-
However, the added silver salts migrate and diffuse to the anode and form metal films on the anode surface that interfere with cell operation.
U.S. Pat. No. 4,416,958 to Miles and Fletcher discloses a thermally activated electrochemical cell having a low melting point electrolyte. The electrolyte is composed of a layer of a mixture of lithium perchlorate and lithium nitrate adjacent the anode and of a layer of a mixture of lithium perchlorate, lithium nitrate, and silver nitrate adjacent the cathode of the cell.
The article titled "Problems Associated with the Electochemical Reduction of Metal Ions in LiNO.sub.3 -KNO.sub.3 and LiClO.sub.4 -KClO.sub.4 Melts" by M. H Miles et al, Journal of the Electrochem. Society, Vol. 134, No. 3, March, 1987, pages 614 to 620, discloses electrochemical studies of certain ions including Cd.sup.++, Pb.sup.++ and In.sup.+++ in molten LiNO.sub.3 -KNO.sub.3, showing the effect of the addition of such ions to the molten nitrate electrolyte.
The article "Cyclic Voltammetric Studies of Nitrato Complexes of Various Metal Ions in Molten LiNO.sub.3 +KNO.sub.3 at 180.degree. C., by M. H. Miles et al, J. Electoanal Chem., 221 (1987) 115-128, discloses addition of various metal ions including Pb.sup.++ and Cd.sup.++ and other ions such as Co.sup.++ and Cu.sup.++ to molten nitrates, and the effect of such additions.
However, neither of the above articles discloses or teaches the application of the principles or concepts that are disclosed in the above articles to thermal batteries, particularly employing lithium or calcium anodes.
One object of the invention accordingly is the provision of an improved thermal electrochemical cell.
Another object is to provide a novel thermal electrochemical cell utilizing low melting electrolytes.
A still further object is the provision of improved thermal electrochemical cells incorporating certain cathode materials in the electrolyte.
Yet another object is to provide thermal electrochemical cells having an oxyanionic electrolyte and containing certain metal salts as cathodic material, to increase the potential of the cell.